High-field ordered and superconducting phases in the heavy-fermion compound PrOs₄Sb₁₂

The filled-skutterudite compound PrOs₄Sb₁₂, the first example of a Pr-based heavy-fermion superconductor, displays superconductivity with T_c 1.85 K and has an effective mass m* 50m_e, where m_e is the free-electron mass. For magnetic fields above 4.5 T, sharp features in the normal-state electrical resistivity, magnetization, specific-heat, and thermal-expansion data suggest the occurrence of a phase transition at high fields. This high-field ordered phase in the normal state may be associated with a combination of crystalline electric fields, Zeeman splitting, and quadrupolar ordering. We present an investigation of the electrical resistivity and magnetization of PrOs₄Sb₁₂ as a function of temperature between 350 mK and 3.5 K and magnetic field up to 18 T. The data reveal a detailed phase boundary of the high-field ordered phase as well as the lower critical field H_c1 and the onset field of the peak effect in the superconducting state of PrOs₄Sb₁₂.